Transducer for providing distance information about a target relative to a transducer

ABSTRACT

A transducer is disclosed which provides distance or position information about a target relative to the transducer. The transducer has first resistor and a first electrode for forming a first capacitive coupling with a target, a second resistor and a second electrode for forming a second capacitive coupling with the target. Signals of opposite phase or polarity are applied to the respective resistor electrode combinations. By applying signals of opposite phase or polarity, no net signal is applied to the target, thereby avoiding problems with cross talk between adjacent transducers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to transducers, and in particular to acapacitance transducer for providing distance or position informationabout a target relative to the transducer.

1. Discussion of Prior Art

Typically, a capacitance position transducer is used to provideinformation about the position (or proximity) of an object (or target)relative to the transducer. A conventional transducer includes a singleelectrode plate for forming a capacitive coupling with the target. Theamount of capacitive coupling is inversely proportional to the distancebetween the electrode plate and the target. The capacitance, and hencethe distance, can be detected by means of an oscillator whose frequencyis dependent on the capacitance value.

A typical oscillator circuit 1 is shown in FIG. 1 of the accompanyingdrawings. The electrode plate 3 is coupled to the inverting input of anoperational amplifier 5. Negative feedback is provided via a firstresistor 7. The positive input of the operational amplifier 5 is fed bya potential divider formed by two resistors 9,11 coupled in seriesacross the output of the amplifier 5.

The transducer operates because the target (not shown) is substantiallyearthed, or is coupled relative to earth. The transducer effectivelyresponds to the relatively small capacitance between the electrode plate3 and the target, in series with the capacitance of the target to earth.

With reference to FIG. 2, a problem can arise if more than one suchtransducer 1 is used in conjunction with a single target 13, forexample, to measure the distances between the transducers 1 and thetarget 13 around the target 13. The problem is caused by cross talkbetween the individual transducer capacitor signals. Further, if C_(T)represents a typical value of each transducer capacitance, and C_(E)represents the target to earth capacitance, then for less than 1% crosstalk the value of C_(E) must be at least one hundred times the value ofC_(T) ; even this factor may not be sufficient in practice.

SUMMARY OF THE INVENTION

The present invention has been devised to overcome problems associatedwith the use of more than one capacitance transducer with a singletarget.

Accordingly, the present invention provides a transducer for providingdistance or position information about a target relative to thetransducer, the transducer comprising a first resistor and a firstelectrode for forming a first capacitive coupling with a target, asecond resistor and a second electrode for forming a second capacitivecoupling with the target, means for applying signals of opposite phaseor of opposite polarity to the respective resistor and electrodecombinations and processing means for processing the voltages appearingacross the electrodes to produce an output indicative of the distance orposition of the target relative to the transducer.

When applying signals of opposite phase or of opposite polarity, no netsignal is applied to the target. Problems associated with cross talkare, therefore, avoided, which is a marked improvement over the priorart transducer mentioned above. Further, a secondary advantage isprovided in that common mode rejection occurs causing the circuit toignore any signal on the target.

Preferably the processing means combine the voltages appearing on thetwo electrodes to produce an output indicative of the distance orposition of the target from the transducer. Other ways of processing thevoltages appearing across the electrodes may, of course, alternativelybe used.

The means for applying signals preferably comprise an oscillator forproducing equal and opposite waveforms to be applied to the respectivefirst and second resistor and electrode combinations.

Preferably a head of the transducer carries the first and secondelectrodes in a first plane and a tail of the transducer substantiallyperpendicular to the first plane is adapted to connect the transducer toa power source.

The processing means may be mounted in the head of the transducer.Further, the processing means may be formed on a printed circuit boardmounted in the head of the transducer behind the first and secondelectrodes. Capacitance between the circuitry of the processing meansand the electrodes may be minimised by, for example, making the plane ofthe circuitry (e.g. the p.c.b.) substantially perpendicular to the planeof the electrodes.

Preferably the head of the transducer tapers from a wide end carryingthe electrodes to a narrow end connected to the tail of the transducer.

The first and second electrodes are preferably substantially rectangularand mounted in an insulator on an exposed surface of the transducer.

Guard rings may be provided arqund the electrodes to guard theelectrodes from any stray signals. More particularly, the guard ringsare included to cancel the effect of capacitance to earth inside thebody of the transducer, thus increasing sensitivity to earth outside.Further, the connections to the electrodes from the signal applicationmeans and from the electrodes to the processing means may also beshielded.

The head of the transducer is preferably made of a conductive material,such as steel or aluminium. The electrodes themselves are preferablymade from copper or brass. Any other appropriate materials can, ofcourse, alternatively be used.

The head of the transducer preferably comprises a body and a cap, thecap being removable to facilitate access to the processing means withinthe head. General assembly of the transducer will also be simplified.

The electrodes may present a curved face for interacting with acorrespondingly curved target. In such an arrangement, the target may bea rotating cylinder, the electrodes being arranged to lie in a linesubstantially parallel to the axis of the cylinder.

In another aspect of the present invention, there is provided anapparatus comprising a rotor, a stator for driving the rotor and aplurality of electromagnets for supporting the rotor relative to thestator, wherein at least one transducer as herein claimed is includedfor providing information on the position of the rotor relative to thestator.

In a preferred arrangement, the rotor is cylindrical and rotates aroundthe stator. In such an arrangement, the transducer or transducers may bepositioned between poles of the electromagnets or poles of the stator.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the present invention are now described, by wayof example only, with reference to the accompanying drawings, in which:

FIG. 1 is a simple example of a prior art transducer circuit asdescribed above;

FIG. 2 is a schematic view of a plurality of prior art capacitancetransducers surrounding a target;

FIG. 3 is a schematic circuit illustrating the principles of the presentinvention;

FIG. 4 is a simple circuit diagram illustrating an arrangement of signalapplication means, processing means and electrodes as found in anembodiment of the present invention;

FIG. 5 is a schematic sectional side view of a head of a capacitancetransducer according to the present invention;

FIG. 6 is a schematic front view of the head shown in FIG. 5; and

FIG. 7 is a schematic plan view of a portion of a stator and rotorapparatus incorporating a transducer according to the present invention.

DETAILED DISCUSSION OF PREFERRED EMBODIMENTS

As can be seen from FIGS. 3 and 4 of the accompanying drawings, acapacitance transducer according to the present invention in itssimplest form comprises a first electrode 21, a second electrode 23, anoscillating input 25 to the resistor conected to the first electrode 21,an oscillating input 27 to the resistor connected to the secondelectrode 23 and connections 29, 31 from the electrodes 21, 23 toprocessing means comprising an operational amplifier 33, 45. Theoscillating inputs 25, 27 to the electrodes 21, 23 are of oppositepolarity so that there is no net resultant signal applied to the target(not shown) which could upset neighbouring transducers via cross talk.Clearly, of course, if the electrodes 21, 23 are at different distancesto the target, a signal may be left on the target, but this signal willbe small if the positions of the two electrodes 21, 23 are controlledcarefully to be equivalent.

In a particular embodiment, each electrode 21, 23 is substantiallyrectangular, the length of each electrode being approximately 20 mm andthe width being 14 mm. A separation of approximately 10 mm between theshorter sides of the electrodes 21, 23 is also provided, as shown inFIG. 6. The sizes are, of course, variable, depending upon theapplication of the capacitance transducer.

As can be seen from FIG. 4, in a particular embodiment the inputs 34,36pass via 100 kΩ resistors 35 to operational amplifiers 37. One input 34is fed into the inverting input of its operational amplifier 37 and theother input 36 is fed into the positive input of its operationalamplifier 37. As a result, the two inputs 25, 27 are of oppositepolarity. Each input 25, 27 is then fed via a 330 kΩ resistor 39 to itsrespective electrode 21, 23, which is protected by a respective guardring 22,24. The voltage across each electrode 21, 23 is applied via anoperational amplifier 41 with negative feedback and a 100 kΩ resistor 43to an operational amplifier 45 which subtracts one output from theother. 100 kΩ resistors 47,49 are positioned between the positive inputto the amplifier 45 and earth and between the output from the amplifier45 and the inverting input of the amplifier 45 respectively. The outputfrom the amplifier 45 passes through another 100 kΩ resistor 51 to theinverting input of an amplifier 53, the positive input of which is fedvia a potential divider formed by two 10 kΩ resistors 55 from the outputof the amplifier 53.

With reference to FIGS. 5, 6 and 7 of the accompanying drawings, thecapacitance transducer 60 comprises a head 61 and a tail 63. Theelectrodes 21, 23 are mounted in an epoxy resin (or any other suitablematerial) insulator 65 at the wider end of the head 61. The electrodes21, 23 and epoxy resin insulator 65 are mounted in a body 67 of the head61 and guarded connections 69 extend from the electrodes 21, 23 to aprinted circuit board 71 mounted within a cavity in the body 67. Theprinted circuit board 71 carries the circuitry for applying opposingsignals to the respective resistor and electrode 21, 23 combinations andfor reacting to voltages received therefrom for processing. A powersupply (not shown) is connected to the printed circuit board 71 via thetail 63 of the transducer 60.

A cap 73 overlays the printed circuit board 71 to close the head 61 ofthe transducer. In this regard, the cap 73 includes side flanges 75which are received in recesses 77 of the body 67 with a tight fit.

As can be seen in FIG. 7, a capacitance transducer 60 according to thepresent invention may be incorporated in an apparatus comprising acylindrical rotor 81 driven about a motor stator (not shown).Electromagnets 83, comprising a plurality of poles 85 carrying coils 87,are included to suspend the rotor 81 about the motor stator to reducefriction losses during rotation of the rotor 81.

The transducer 60 may be provided with two legs 63 (cf. FIG. 6), ratherthan a single tail 63, which straddle the core 89 of the electromagnets83 as shown in FIG. 7. The position of the transducer 60 is therebyaccurately positioned during assembly of the complete apparatus.Further, by virtue of the transducer 60, the position of the rotor 81about the motor stator and electromagnets 83 can be accuratelydetermined. In this regard, the rotor 81 may be within a few millimetres(perhaps even 1 mm) of the faces of the poles 85 during use. As aresult, the electrodes 21, 23, which are set back relative to the polefaces to ensure that the rotor 81 strikes the pole faces rather than thetransducer 60 in the event of a failure, have curved surfaces to conformwith the curved inside surface of the rotor 81. Such an apparatus may bean energy storage and conversion apparatus, wherein energy is stored inthe rotor 81 as kinetic energy of the rotor 81 by means of the motorstator (not shown) driving the rotor 81 and removed therefrom when themotor stator and rotor 81 act as a generator to withdraw energy.

Although an apparatus has been described wherein the rotor 81 is outsidethe motor stator, a transducer 60 according to the present inventioncould alternatively be used in connection with a rotor mounted within astator or, indeed, in any other appropriate apparatus.

It will of course be understood that the present invention has beendescribed above purely by way of example, and that modifications ofdetail can be made within the scope of the invention.

We claim:
 1. A transducer for providing distance or position informationabout a target relative to the transducer, the transducer comprising:afirst resistor and a first electrode for forming a first capacitivecoupling with the target, a second resistor and a second electrode forforming a second capacitive coupling with the target, means for applyingsignals of one of opposite phase and of opposite polarity to therespective resistor and electrode combinations, and processing means forprocessing the voltages appearing across the first and second electrodesto produce an output indicative of one of distance and position of thetarget relative to the transducer.
 2. A transducer as claimed in claim1, wherein the means for applying signals comprise an oscillator forproviding equal and opposite waveforms to be applied to the respectivefirst and second resistor and electrode combinations.
 3. A transducer asclaimed in claim 1, wherein a head of the transducer carries the firstand second electrodes in a first plane and a tail of the transducersubstantially perpendicular to the first plane is adapted to connect thetransducer to a power source.
 4. A transducer as claimed in claim 3,wherein the processing means are mounted in the head of the transducer.5. A transducer as claimed in claim 4, wherein the processing means areformed on a printed circuit board mounted in the head of the transducersubstantially perpendicular to the first and second electrodes.
 6. Atransducer as claimed in claim 3, wherein the head of the transducertapers from a wide end carrying the electrodes to a narrow end connectedto the tail of the transducer.
 7. A transducer as claimed in claim 1,wherein the first and second electrodes are substantially rectangularand mounted in an insulator on an exposed surface of the transducer. 8.A transducer as claimed in claim 1, wherein guard rings are providedaround the electrodes.
 9. A transducer as claimed in claim 1, whereinthe transducer comprises a body and a cap, the cap being removable tofacilitate access to the processing means within the body.
 10. Atransducer as claimed in claim 9, wherein the body and cap of thetransducer are made of aluminium.
 11. A transducer as claimed in claim1, wherein the first and second electrodes present a curved face forconforming with a correspondingly curved target.
 12. An apparatuscomprising a rotor, a stator for driving the rotor and a plurality ofelectromagnets for supporting the rotor relative to the stator, whereinat least one transducer according to claim 1 is included for providinginformation about the position of the rotor relative to the stator. 13.An apparatus as claimed in claim 12, wherein the information provided bythe transducer is used to control the electromagnets.
 14. An apparatusas claimed in claim 12, wherein the rotor is cylindrical and rotatesabout the stator.
 15. An apparatus as claimed in claim 12, wherein thetransducer is positioned between poles of the stator or poles of theelectromagnets.
 16. An apparatus as claimed in claim 12, wherein theelectrodes are arranged to lie in a line substantially parallel to theaxis of the rotor.
 17. An apparatus as claimed in claim 12, wherein theapparatus is an energy storage and conversion apparatus.
 18. Atransducer for providing distance or position information about a targetrelative to the transducer, the transducer comprising:a first resistorand a first electrode for forming a first capacitive coupling with thetarget, a second resistor and a second electrode for forming a secondcapacitive coupling with the target, an oscillator applying signals ofone of opposite phase and of opposite polarity to the respectiveresistor and electrode combinations, and a comparator responsive tovoltages appearing across the first and second electrodes producing anoutput indicative of any difference in capacitance coupling between saidelectrodes and said target, said difference representative of one ofdistance and position of the target relative to the transducer.
 19. Anapparatus comprising:a rotor, a stator for driving the rotor, and aplurality of electromagnets for supporting the rotor relative to thestator, wherein at least one transducer according to claim 19 isincluded for providing information about the position of the rotorrelative to the stator.